In certain manufacturing processes, it is necessary to provide heat transfer between combined liquid and vaporous phases of a heat exchange fluid, and a feed fluid. For efficient operation and optimum heat transfer, the liquid and vapor should be uniformly distributed across the width of the heat exchanger prior to passing in heat exchange relationship with the feed fluid. Apparatus and a method for accomplishing this are disclosed in U.S. Pat. No. 3,559,722, assigned to the same assignee as the present invention.
In the '722 patent, transfer passage means provide fluid communication between the liquid and vapor passages and are disclosed as a slot in the metallic plates separating the liquid and vapor, extending the width of the heat exchanger. Since the heat exchanger structure is weakened by the slot in the plates, a corrugated sheet metal fin is shown "bridging" the slot to support the metallic plates on each side of the slot. The corrugations of the "bridging" fin are aligned parallel to the longitudinal axis of the heat exchanger and do not significantly inhibit fluid flow through the transfer passage means. Upstream of the "bridging" fin, another rectangular-shaped corrugated fin structure is disposed with the corrugations extending across the fluid flow path, so that the fluid is forced to flow through perforations in the fin walls, in the "hard" way. These "hard way" fins improve the lateral distribution of the fluid in the passages wherein they are disposed and partially restrict fluid flow through the heat exchanger in accordance with design criteria.
An alternative prior art design uses sparge tubes (conduit having a plurality of perforations therein) to distribute one of the fluid phases across the width of the heat exchanger prior to admitting it into flow passages in which the other heat exchange fluid has been distributed. An example of this type heat exchanger is disclosed in U.S. Pat. No. 3,895,676. The sparge tube heat exchanger typically is used for moderate two phase fluid flow rate applications at low pressure, e.g., less than 250 psi, although it can be built to operate at higher pressures, in excess of 700 psi. By comparison, the maximum pressure rating of a typical heat exchanger built according to the '722 patent is about 525 psi. The "bridging" fins and "hard way" fins used in the split parting plate heat exchanger limit the structural strength and subsequently, the pressure rating of that type heat exchanger.
Heat exchangers of the type cited operate efficiently only at specific mass flow ratios of the liquid and vaporous phases. Although such heat exchangers may operate properly when the flow rates of both the liquid and vapor change by the same percentage, they are generally inefficient in coping with significant changes in the ratio of liquid flow to vapor flow. For example, a significant increase in the liquid flow may flood or "drown" the vapor distribution means, thereby preventing proper distribution of the vapor across the heat exchanger prior to mixing with the liquid. Heat exchangers of prior art design have not provided means to meet the requirements of processes in which the ratio of liquid to vapor flow may change substantially, as for example during start-up and shut-down, or during operation under stable temperature conditions which prevent the mass flow ratio from reaching equilibrium. The ratio of liquid to vapor flow cannot be controlled over more than a very narrow range by means external to the heat exchanger without interfering with the efficient distribution and mixing of the liquid and vapor fluids internal to the heat exchanger.
In consideration of the above problems, the present invention provides the means to extend the pressure rating of a split plate-type heat exchanger to a level over 700 psi, and the means to control the ratio of liquid to vapor flow through the heat exchanger over a much wider range than previously available.